Reading :help 'tags' and scrolling a little bit down finds the following sentence:
The *tagfiles()* function can be used to get a list of the file names
actually used.
Accordingly, :echo tagfiles() will show the name(s) of found tagfile(s).
An easy way to test this is in any of Vim's help files.
It will pick up the helptags in <whatever>/doc/tags.
